1. Field of the Invention
This invention relates to a power converter apparatus including a chopper and an inverter, and more particularly to a power converter apparatus which utilizes as a DC power source a solar battery or the like and which is therefore provided with large fluctuations in voltage.
2. Discussion of Background
In general, a solar battery is widely utilized for a power supply system that supplies specified power through a power converter apparatus constituted by a chopper and an inverter to an independent load which can be another power system. Such a solar battery has voltage-current and power characteristics as shown in FIG. 1 which characteristics differ as a function of the amount of incident solar energy. In FIG. 1, the voltage-current characteristics are shown by solid lines, and the power characteristics are shown by dotted lines. The respective characteristics indicate a tendency for both the current and power to increase as the amount of incident solar energy increases.
As can be seen from the characteristics shown in FIG. 1, a solar battery has the disadvantage that, due to its large fluctuations in voltage, power to be supplied decreases when an excessive amount of current is utilized. However, although the power to be supplied varies drastically depending upon the amount of incident solar energy, the battery voltage which produces maximum power outputs is constant irrespective of the amount of incident solar energy.
On the other hand, a power converter apparatus has been designed as a self-commutated inverter in order to receive maximum power from a solar battery, with the maximum power obtained in correspondance with the respective amounts of incident solar energy so as to supply the same to another power system. This is because when operating cooperatively with another power system which cannot be controlled, the output voltage and phase of the self-commutated inverter apparatus should be controlled.
The conventional converter apparatus is constituted by a chopper and an inverter. Installation of the chopper at the DC input of the inverter allows the chopper to perform a constant output voltage control, and the inverter to control the output pulse width to a constant value, whereby the harmonics components in the output voltage of the inverter are reduced.
In the power converter system that utilizes the solar battery for a power source, there is substantially no period throughout the year in which the power converter apparatus receives a maximum power output from the solar battery, i.e., the period to perform a full load operation, and the average load factor thereof throughout the year is approximately 20 to 80%, so that the efficiency of the power converter during light-load operation apparatus is regarded as of major importance. In addition, the solar battery currently has an extremely high manufacturing cost per kw, so that it is particularly significant to reduce the loss of the power converter apparatus as compared to the system that utilizes another power source such as a storage battery.
However, in the conventional power converter apparatus, as described above, installation of the chopper prior to the inverter stage is advantageous because the output filter circuit can be made compact and the control circuit simplified. On the other hand, disadvantages arise in that the loss developed by switching the DC circuit with the chopper is not reduced even during light-load operation.
In general, the total loss Pt of the switching elements employed within the chopper can be expressed by the following equation: EQU P.sub.t =(P.sub.on +P.sub.off).times.f+P.sub.s ( 1)
where P.sub.s represents the loss in a steady ON state, P.sub.on a switching loss when turned on, P.sub.off a switching loss when turned off, and f a switching frequency, respectively. In equation (1), the steady ON state loss P.sub.s of the second term is proportional to a steady ON current. However, the first term ((P.sub.on +P.sub.off).times.f) is a component proportional to the switching frequency of f. Namely, the loss of the chopper can be separated into a fixed loss of the first term and a load loss of the second term. This results in the disadvantage that when the power converter apparatus is in light-load operation, the proportion of the fixed loss becomes greater, whereby the efficiency of the power converter apparatus becomes lowered. Moreover, additional disadvantages exist in that should the switching frequency f be lowered in order to reduce the fixed loss without due consideration for its various aspects, i.e., without consideration of the load, the quick-response characteristics of chopper control are drastically deteriorated. Further, the DC reactor required for such a chopper is exceptionally bulky, whereby the economic advantages and compactness of the power converter apparatus are also adversely affected.